Ultrasound imaging works by using high frequency sound waves and their echoes to obtain images inside the human body. A transducer probe is used to generate the sound pulses and transmit them into the body.
The sound waves travel into the body and are strongly reflected at interfaces between different types of tissue such as fat and muscle, or muscle and bone. At each interface a fraction of the sound wave is reflected and the rest transmitted through the interface to penetrate further into the tissue. This process occurs at each interface and by recording the reflected sound wave echoes an image can be produced. The reflections at the interfaces arise due to the impedance mismatch between different layers of the tissue. For instance, the impedance of the fat layer is different from that at the fat-muscle interface. This property is made use of to calculate the thickness of the tissue.
Fetal heart rate monitoring utilizes Doppler ultrasound to detect signs of fetal distress, especially in high risk patients and during labor. Current ultrasound transducers emit a narrow cylindrical ultrasound beam to detect and record the heartbeat and so have a limited and constricted fetal heart detection range. Their effectiveness is inhibited by limited detection range, patient movement, and bulkiness, so one pertinent clinical issue is the frequent readjustment of the traditional ultrasound transducer by nurses during labor. Due to shifting of the fetus or mother during birthing, the current devices can often lose the heartbeat.
During labor and delivery, the fetal heart rate is monitored by ultrasound and the strength, duration and length of uterine contractions is monitored electronically with a device called a tocometer. For a normal delivery, ultrasound can be used to monitor the baby's heartbeat externally. A normal heart rate indicates that the fetus is receiving sufficient oxygen throughout the contractions. While the fetal heart rate changes in response to labor contractions, erratic changes to fetal heartbeat during the birthing process, can indicate labor complications that may require emergency care. Since current ultrasound transducers produce parallel beams of ultrasound, approximately six cm in diameter, the personnel monitoring the delivery has to continuously change position of the current ultrasound transducers as the baby moves.